Broadband X-ray generation by large krypton clusters under the effect of relativistic femtosecond laser pulses

This paper presents a phenomenological analysis of thermodynamic parameters of a pulsed supersonic gas jet in a conical nozzle. Conditions have been found out for the condensation and formation of large krypton clusters resulting from phase transitions during adiabatic expansion of gas through the nozzle into vacuum. We have identified the phase state of the large clusters (submicron droplets consisting of ~10⁷ monomers) forming in the jet and obtained a high yield of X-rays from a cluster nanoplasma produced by femtosecond laser pulses of relativistic intensity I. It has been shown that, at I ≈ 3 × 10¹⁸ W cm^-2, a nanoplasma consisting of large krypton clusters emits a broadband X-ray spectrum in the range 5 – 100 keV. The integrated laser to X-ray energy conversion efficiency achieved is ~10^-5 (400 nJ), and the efficiency of characteristic Kα line (12.6 keV) generation is ~1.5 × 10^-6 (68 nJ), which corresponds to a quantum yield of ~3 × 10⁸ photons s^-1 (4π)^-1, with a contrast of ~25 relative to bremsstrahlung background.